1. Field of the Invention
The field of art to which this invention pertains is the solid bed adsorptive separation of isomeric toluenediamines. More specifically, the invention relates to a process for separating 2,4-toluenediamine from 2,6-toluene diamine and other toluenediamine isomers by employing a solid bed adsorption system.
2. Background Information
The isomers, 2,4-toluenediamine and 2,6-toluenediamine are important precursers of polyurethanes which are useful in many applications as rigid or flexible forms or as fibers, e.g., insulation, soundproofing, interlinings for clothing and sleeping bags, cushions, spandex, etc.
It is common industrial practice to make polyurethane from a mixture of the isomers, 2,4- and 2,6-toluene diisocyanate (TDI), for example 80/20 or 65/35, derived from mixtures of 2,4- and 2,6-toluenediamine, because it is difficult and expensive to separate them by existing techniques. Current methods of separating the isomers involve crystallization, e.g. U.S. Pat. No. 4,246,187, and hence, are energy-intensive and time-consuming. Moreover, polyurethanes derived from pure 2,4-toluenediamine have dramatically different properties compared to materials synthesized from mixtures. It is further expected that polyurethanes synthesized from (relatively) pure 2,6-toluenediamine will have different and more useful properties, for example, a higher glass transition temperature, Tg, and greater stability. Also, the reaction speed with a single isomer is expected to be more consistent. Accordingly, it is desirable to separate the TDI isomers or their precursors, the toluenediamines, by an economical process.
It is well known in the separation art that certain crystalline aluminosilicates can be used to separate hydrocarbon types from mixtures thereof. Furthermore, X and Y zeolites have been employed in a number of processes to separate individual hydrocarbon isomers.
It is known from U.S. Pat. No. 3,069,470 to Fleck et al, to use type X zeolites for the separation of the meta isomer from other isomers of toluidine. In U.S. Pat. No. 4,480,129 to Priegnitz et al, it is disclosed to separate p-toluidine from its isomers with an X- or Y- type zeolite exchanged with Fe, Mn, Co, Ni or Zn and a desorbent comprising aniline or, when using an X-type zeolite, alkylamine having greater than eight carbons.
It is known from U.S. Pat. No. 4,270,013 to Priegnitz et al that ortho-nitrotoluene may be separated from other nitrotoluene isomers by using a type X zeolite containing at exchangeable cationic sites one cation selected from a group that includes potassium and barium. The specific desorbent materials disclosed by this reference are toluene and 1-hexanol. The separation of isomers of disubstituted benzenes with crystalline aluminosilicates having silica/alumina mole ratio of at least 12 is disclosed in U.S. Pat. No. 4,467,126 to Zinnen.
In copending application Ser. No. 782,674 filed Oct. 1, 1985, the adsorptive separation of another precursor of polyurethane, 2,4-dinitro toluene and 2,6-dinitrotoluene is disclosed in which the separation is achieved by an L, X or Y type zeolite cation exchanged with K, Na, Ca, Ba, Li or Mg and desorbed by a C.sub.3 to C.sub.5 alcohol, a ketone, ester or a nitrocompound.
Also, in copending application Ser. No. 782,345 filed Oct. 1, 1985, the separation of minor isomers, 2,3- and 3,4-dinitrotoluene, from the major isomers, 2,4- and 2,6-dinitrotoluene, with a calcium or sodium exchanged Y type zeolite is disclosed. The desorbent can be an ester, an alcohol or a ketone. Presence of the minor isomers lowers the yield of polyurethane precursors and produces high molecular weight side products.
Alternatively, the final precursor of polyurethane, toluene diisocyanate, is normally available as mixtures of isomers. The invention in copending application Ser. No. 781,561 filed Sept. 30, 1985 is the separation of 2,4- and 2,6-diisocyanate (TDI) by adsorption onto a type Y zeolite exchanged with K, Ca, Na, Li, H, and Mg and desorbed with toluene. It is hypothesized that polyurethanes made with a single, pure isomer of TDI may exhibit improved properties.